1. Field
The present invention relates to drives for rotary distributors for wastewater filtration systems.
2. State of the Art
Trickling filter systems, which use a layered filter bed of stones, have been employed as one of the principal stages of sewage treatment for many years. In recent years, a plastic filter media, sometimes of a honeycomb design, has replaced stone in the filter bed. The raw sewage liquor is sprinkled over the top of the bed by a distribution system including pipes (also called tubes or arms) extending from a centerwell as horizontal rotating arms which discharge the sewage or other waste liquor through spaced ports distributed along their length. Systems may use double or multiple arms comprised of long tubes supported by overhead tie-rods and spaced apart by horizontal tie-rods. The distributor pipes are typically fed from their radially inward ends through a centerwell or mast. As the distribution process occurs, the liquor is discharged horizontally from discharge ports located on the same side of each of the distribution pipes. As a result, at substantial flow rates, an inertial reaction of the distribution pipes to the fluid flow jetting out behind the pipe causes the distribution system to rotate. By proper placement of the discharge ports, a distribution system can be designed to distribute the liquor evenly over the top of the bed. As the liquor passes down through the filter bed, biological degradation processes the sewage and liquifies the suspended so ids.
As a distributor system operates, the flow of liquid through the system is subject to several variations. The total volumetric flow rate may fluctuate with time between reasonably high flows and extremely low flows. Since the motion of the distributor pipes or arm is dependent upon the jetting action of the exiting liquid, at low flows the jetting may be insufficient to overcome the friction within the system. If the system stops, the friction changes from a dynamic coefficient of friction to a static coefficient of friction. Static coefficients of friction are typically larger than dynamic coefficients of friction, so a system which has ceased to rotate is more difficult to restart. There is a need for a support system which will insure the rotation of a sewage distribution system whenever extremely low flow conditions result in insufficient hydraulic forces to overcome the system frictional forces.
During the time that a distribution system is not properly rotating, wastewater treatment is not effectively taking place. In the past, the operators of wastewater distribution systems which failed, due to malfunction, low flow rates or other reasons, might simply pass their discharges through the bed within a very small area and thus not provide the biological degradation which constitutes sewage treatment processes. In previous decades, such a failure could be ignored or might have been ignored. Today, stricter administrative regulation of the quality of discharge of untreated wastewater effectively precludes a municipality or industrial treatment plant from discharging untreated sewage or inadequately treated sewage. Fines and other civil and criminal sanctions for violations require that some means be developed to maintain proper operation of treatment systems with minimal attendance by operators.
Moreover, as populations in some urban centers increase, with the attendant load increases on wastewater treatment plants, a desire for optimized throughput through each existing filter bed becomes more desirable to avoid the necessity for new installations. Also, real estate purchased and dedicated to treatment facilities becomes more expensive, if not impossible, to enlarge in population centers where cities have built up around their treatment plants. That is, if a treatment plant has limited existing real estate, it is desirable to minimize costs by stacking more filter beds vertically or by placing them in closer proximity, in a single treatment works to avoid the costs of land acquisitions. Such an approach, however, requires more system sophistication and enhanced monitoring to prevent failure of any individual filtering bed. Thus, enhanced system reliability has economic advantages in allowing more treatment beds to be installed within less area and operated by fewer people per unit and also provides the ability to increase system throughput and reduce system down-time.
As noted above, wastewater treatment systems of the trickling filter or filter bed variety use a rotating array of distribution pipes to spread liquor over the top of the filter bed. Two means have been developed for rotating these distribution systems or networks. The first uses the jetting action of the fluid being discharged to cause (via inertial reaction) horizontal rotation of the "star" of radial distribution pipes or arms turning about a center mast, whereas the second uses a motor operating either from the center of the star or along a track at the periphery of the star to cause rotation. The difficulty with the former approach in the modern age is lack of reliability when distribution arms or discharge ports become plugged or when discharge rates become so low as to limit rotation speeds or bring the system to a stop. The difficulty with the latter mechanism is the inordinate cost of driving the star. At the slow rotational speeds of such stars, as the distribution pipes discharge to the filter bed, gearing and mechanical losses can be substantial, causing a significant power cost for system rotation in addition to pumping costs.
Finally, the reliability of a motorized drive system utilized as the primary power source is not necessarily better than that of a hydraulically driven (jetting or inertial reaction) system. Mechanical failures can occur in motor drives and gear trains as well as in hydraulic systems. Thus, a need exists for a reliable system which provides minimal operating costs. Little development has occurred for decades in this area of wastewater treatment equipment.
A typical hydraulically driven trickling filter sewage distribution systems is disclosed in U.S. Pat. No. 2,403,695 by Walker. The sewage is pumped to and through a central distribution pedestal from which numerous arms or tubes radiate, and is then distributed out through the arms where it is discharged horizontally through specifically spaced discharge openings out over the filter bed. The inertial reaction of the rotating distribution tubes to the discharge velocity of the liquor drives the rotary distributor about its axis at the center of the pedestal. At low flows or with obstructions, friction halts rotation of such a system. One motorized type of drive system is typically described by Hartley, U.S. Pat. No. 904,325. In the Hartley device, a monorail was anchored on the top of the outer wall of the filter bed. A motor at the end of a distribution tube travels along the rail, slowly rotating the array of tubes. No hydraulic benefit is achieved from the discharge of the sewage liquor from the distribution tubes. The advantage sought by Hartley was a positive drive achieved at the cost of continuous operation of an electrical drive mechanism. Other motorized systems in the prior art, as disclosed in FIG. 5 of the Walker patent, employ a motor at the pedestal in the center of the distribution system. In either arrangement, experience in the art has shown such systems to require substantial maintenance, which still cannot prevent periodic breakdowns due to the continual stress placed on the drive system components.